It is known to coat electrically conducting articles with resinous binders containing tertiary amino groups by means of cathodic electrodeposition. Generally such articles are immersed in an aqueous coating composition comprising the resinous binder in the form of a salt thereof and a cross-linking agent, and an electrical current passed between the article (cathode) and an anode which deposits the resinous binder on the article which is then stoved to cure or cross-link the resinous binder.
Known resinous binders having tertiary amino groups may be represented by the formula: EQU A.sub.1 --B.sub.1 --C.sub.1 --B.sub.1 --.sub.m A.sub.1 (I)
wherein m is 0 or an integer of from 1 to 6,
Each A.sub.1, is the same or different tertiary amino group, PA0 B.sub.1, or each B.sub.1 which may be the same or different, is a group of formula: ##STR1## wherein n is 0 or an integer of from 1 to 10, and R is a dihydric phenol residue, and PA0 C.sub.1, or each C.sub.1 which may be the same or different, is a group derived from a compound having at least two sites capable of reacting with glycidyl ether groups. PA0 each A is the same or different tertiary amino group, PA0 B, or each B which may be same or different, is a group of formula: ##STR2## wherein n is 0 or an integer of from 1 to 10, and R is a dihydric phenol residue; and PA0 C, or each C which may be the same or different, is a group derived from a compound having at least two sites capable of reacting with glycidyl ether groups, PA0 (A) a glycidyl ester of formula VI, or PA0 (B) a cyclic carboxylic anhydride and a glycidyl ester of formula VI. PA0 (A) a glycidyl ester of formula VI, and PA0 (B) a compound having at least three sites capable of reacting with glycidyl groups wherein the amounts of (A) and (B) are such that the reaction product has at least two sites capable of reacting with glycidyl ether groups. PA0 (i) water, PA0 (ii) a resinous binder of formula III, PA0 (iii) an acid in an amount sufficient to neutralize at least about 30% of the amino groups of the resinous binder, and PA0 (iv) a water-miscible corss-linking agent.
Resinous binders of this type, wherein C.sub.1 is derived from a diamine or a primary monoamine, are known from U.K. Pat. No. 1,461,823. A disadvantage of such binders is that they produce rough, incoherent coatings having poor corrosion resistance on bare steel substrates, i.e., steel which has not been phosphated. It has also been proposed to incorporate residues of unsaturated fatty acids into such resinous binders, e.g., see U.K. Pat. No. 1,307,585. Although such binders form smoother coatings on bare steel substrates they still have a poor corrosion resistance.
It has now found that if such binders contain at least one group derived from a glycidyl ester of a C.sub.6 to C.sub.20 carboxylic acid then the coatings prepared therefrom are smooth and glossy and have good corrosion resistance even when deposited upon bare steel substrates.